1. Field of the Invention
This invention relates generally to a current perpendicular to plane random access memory (CPP-MRAM) cell formed as a magnetic tunneling junction (MTJ) using a spin transfer effect to change the magnetization of a free layer.
2. Description of the Related Art
The conventional magnetic tunneling junction (MTJ) device is a form of ultra-high magnetoresistive (MR) device in which the relative orientation of the magnetic moments of parallel, vertically separated magnetized layers, controls the flow of spin-polarized electrons tunneling through a very thin dielectric layer (the tunneling barrier layer) formed between those layers. When injected electrons pass through the upper layer they are spin polarized by interaction with the magnetic moment of that layer. The majority of the electrons emerge polarized in the direction of the magnetic moment of the upper layer, the minority being polarized opposite to that direction. The probability of such a polarized electron then tunneling through the intervening tunneling barrier layer into the lower layer then depends on the availability of quantum states within the lower layer that the tunneling electron can occupy. This number, in turn, depends on the magnetization direction of the lower electrode. The tunneling probability is thereby spin dependent and the magnitude of the current (tunneling probability times number of electrons impinging on the barrier layer) depends upon the relative orientation of the magnetizations of magnetic layers above and below the barrier layer. The MTJ device can therefore be viewed as a kind of multi-state resistor, since different relative orientations (e.g. parallel and antiparallel) of the magnetic moments will change the magnitude of a current passing through the device. In a common type of device configuration (“spin filter”), one of the magnetic layers has its magnetic moment fixed in direction (pinned) by exchange coupling to an antiferromagnetic layer, while the other magnetic layer has its magnetic moment free to move (the free layer). The magnetic moment of the free layer is then made to switch its direction from being parallel to that of the pinned layer, whereupon the tunneling current is large, to being antiparallel to the pinned layer, whereupon the tunneling current is small. Thus, the device is effectively a two-state resistor. The switching of the free layer moment direction (writing) is accomplished by external magnetic fields that are the result of currents passing through conducting lines adjacent to the cell.
FIG. 1 is a highly schematic drawing showing an overhead view of a conventional MRAM cell between orthogonal word (200) and bit (100) lines. The cell (10) is drawn with a slightly elliptical horizontal cross-section because such a shape anisotropy produces a magnetic anisotropy within the free layer that assists its magnetic moment in retaining a thermally stable fixed position after switching fields have been turned off. According to the diagram, the word line field will be along the short or “hard” axis of the cell, the bit line field will be along the easy axis (the longer axis of the ellipse) along which the magnetic moment of the cell spontaneously forms.
For MRAM applications, the MTJ element is usually formed with shape anisotropy as shown in FIG. 1. Shape anisotropy occurs when the horizontal cross-sectional shape deviates from circular and becomes elliptical. When such an elliptical cell is in its quiescent state (not being activated by a switching current), the magnetization of the free layer lies along the longer (easy) axis of the cell. Within this easy axis direction, the free layer magnetization can be directed along the magnetization direction of the pinned layer or opposite to it, i.e., either parallel or anti-parallel to the pinned layer magnetization. In the parallel mode, the cell resistance is minimum, in the anti-parallel mode the cell resistance is maximum. Thus, the storage of digital information is provided by the magnetization direction of the free layer. The field required to switch the free layer magnetization from parallel to anti-parallel mode is called the switching field, Hs, and its value is a function of the magnetic anisotropy energy of the cell element.
In the conventional (non-spin transfer) MRAM application, two orthogonal external fields are used to program the MRAM cell. These fields are provided by the current carrying bit and word lines between which the cell is positioned. Referring to FIG. 2, there is shown schematically an array of cells (10) positioned at the intersections of orthogonally directed word (200) and bit (100) lines. The easy axes of the cells are along the direction of the word lines, so it is the magnetic field produced by the bit lines that is along the direction of the easy axis, while the magnetic field of the word line is along the perpendicular hard axis. To switch the magnetization direction of a selected cell, both fields are required to be “on” at the position of the selected cell. The combination of the two fields will be sufficient to overcome the energy of the shape anisotropy that maintains the cell magnetization in its quiescent direction. In the figure, arrows indicate the particular word and bit lines that are carrying current. The cell (101), shown darkened, that is at their intersection is the selected cell and its magnetization will be switched. However, there are many cells that lay beneath the active bit and word lines that are still experiencing partial fields, though, in principle, fields that are insufficient to achieve the required switching. Those cells can still, however, be accidentally programmed, causing an error. Such cells are referred to as being half-selected and are shown as shaded in the figure. The fact that such an array of crossed word and bit lines can produce unwanted cell switching is a problem associated with the use of externally generated magnetic fields to switch MRAM cells. Another problem associated with externally generated magnetic fields is the problem of scaling, which is to say that as the cells become smaller so must the current carrying lines and this requires the passage of higher currents through lines of greater resistance, increasing the power consumption of the circuit.
For this reason, a new type of magnetic device, called a spin transfer device and described by Slonczewski, (U.S. Pat. No. 5,695,164) and by Redon et al. (U.S. Pat. No. 6,532,164) has been developed and seems to eliminate some of the problems associated with the excessive power consumption necessitated by external switching fields. The spin transfer device shares some of the operational features of the conventional MTJ cell described above, except that the switching of the free layer magnetic moment is produced by the spin polarized current itself. In this device, unpolarized conduction electrons passing through a first magnetic layer having its magnetic moment oriented in a given direction (such as the pinned layer) are preferentially polarized by their passage through that layer by a quantum mechanical exchange interaction with the polarized bound electrons in the layer. Such a polarization can imposed on conduction electrons that reflect from the surface of the magnetized layer as well as to those that pass through it. When such a stream of polarized conduction electrons subsequently pass through a second magnetic layer whose polarization direction is not fixed in space (such as the free layer), the polarized conduction electrons exert a torque on the bound electrons in the magnetic layers which, if sufficient, can reverse the polarization of the bound electrons and, thereby, reverse the magnetic moment of the magnetic layer. If the magnetic moment of the layer is directed along its easy magnetic axis, the required torque is minimized and the moment reversal occurs most easily. The use of a current internal to the cell to cause the magnetic moment reversal requires much smaller currents than those required to produce an external magnetic field from adjacent current carrying lines to produce the moment switching. Recent experiments confirm earlier theoretical predictions (J. C. Slonczewski, J. Magn. Mater. 159 (1996) LI, M. Hosomi et al., IDEM, 2005 and L. Berger, Journ. Appl. Phys., 93 (2003), 7693).
Referring to FIG. 3, there is shown a schematic illustration of an exemplary prior art STT-MTJ element being contacted from above by a bit line (100) and from below by a bottom electrode (300). Moving vertically upward, there is shown a seed layer (1), an antiferromagnetic pinning layer (2), a synthetic antiferromagnetic (SyAF) pinned reference layer (345), consisting of a first ferromagnetic layer (3), a non-magnetic spacer layer (4) and a second ferromagnetic layer (5), a non-conducting tunneling barrier layer (6), a ferromagnetic free layer (7) and a non-magnetic capping layer (8). Arrows, (20) and (30), indicate the antiparallel magnetization directions of the two ferromagnetic layers (3) and (5) of the SyAF pinned layer (345). The double-headed arrow (40) in layer 7 indicates that this layer is free to have its magnetic moment directed in either of two directions.
Referring again to FIG. 3 it is noted that when a critical current (arrow (50) is directed from bottom to top (layer (1) to layer (8)), the free layer magnetization (40) would be switched to be opposite to the direction of the reference layer's magnetization (30) by the spin-transfer torque. This puts the MTJ cell into its high resistance state.
Conversely, if the current is directed from top to bottom (60), the free layer magnetization (40) would be switched, by torque transfer of angular momentum, to the same direction as that of the pinned reference layer (30), since the conduction electrons have passed through that layer before entering the free layer. The MTJ element is then in its low resistance state.
Referring again to FIG. 3, there is shown some additional circuitry, specifically a transistor (500) to inject current into the cell element whenever the cell element is selected to be written upon. The transistor is electrically connected to the cell through a conducting via (80) which allows a current to pass vertically between the bottom electrode (300) and the bit line (100). The word line (200), which can contact the transistor gate, activates the transistor so as to inject the writing current. In this way one can create a single spin-RAM memory cell that utilizes the spin transfer effect (denoted hereinafter as an STT-RAM) for switching an MTJ type element. In this paper, we will use the term “element” to describe the basic MTJ structure comprising a tunneling barrier layer sandwiched between ferromagnetic fixed and free layers. We shall use the term “memory cell” to denote the combination of the MTJ element incorporated within circuitry that permits the element to be written on and read from. The word line provides the bit selection (i.e., selects the particular cell which will be switched by means of a current passing through it between the bit line and the source line) and the transistor provides the current necessary for switching the MTJ free layer of the selected cell. Although it is not shown in this simplified figure, the cell is “read” (i.e., its resistance value is determined) by applying a bias voltage between the bit line and source line, thereby measuring its resistance and comparing that resistance with a standard cell in the circuit.
The critical switching current is given by:Ic=CMs2V,where C is a constant, Ms is the spontaneous magnetization of the free layer and V is the cell volume, V=At, where A is the horizontal cross-sectional area of the cell and t is its thickness. From this equation it is clear that the switching current scales down with the cell dimensions. However, as the dimensions become smaller, thermal agitation may perturb the stored information (i.e., change the magnetization of the free layer). The effect of thermal agitation is given by the equation:f=f0exp{−BHsMsV/kT}, where f is the thermal switching frequency, f0 and B are constants, k is Boltzmann's constant and T is the temperature. For the stored information to be thermally stable (low switching frequency, f) the numerator of the exponentiated fraction, BHsMsV, must exceed a certain constant value. As the dimensions of the cell scale down, the area factor, A, in the volume V=At decreases, so to maintain the value of the numerator, Hs and/or Mst must increase. But increasing Mst will increase Ic, which is undesirable. So increasing Hs is a better choice. Usually Hs is provided by the shape anisotropy of the cell, for example, for an elliptical shaped cell Hs=DMstAr, where D is a constant and Ar is the aspect ratio of the ellipse, the ratio of the long axis to the short axis. To utilize the shape anisotropy, the value of Hs can be increased either by increasing Mst or As. Neither of these is desirable, since increasing Mst increases Ic and increasing Ar will increase cell size.
Clearly a method is needed to increase Hs without encountering the disadvantages of either increasing Ic or increasing cell size. A way of doing this can be provided by utilizing the combination of two phenomena that affect the magnetic anisotropy of the free layer of an MTJ cell: 1) an anisotropy that results from stress and magnetostriction of the free layer that, in turn, is a function of its material composition, its method of formation and the effects of its surroundings, and 2) the effects of shape-induced anisotropy that gives rise to a magnetic anisotropy. The prior art has applied the effects of magnetostriction to controlling the properties of spin-valve type GMR devices. The following are prior art patents and published applications that teach these effects.
Fukuzumi (U.S. Pat. No. 6,970,376), Engel et al. (US Pub. Patent Application 2002/0117727), Iwata (US Pub. Patent Application 2006/0118842), Pukala et al. (US Pub. Patent Application 2005/0106810) and Fukuzumi (US Pub. Patent Application 2005/0104102) discuss the control of magnetostriction and stress.
Fukumoto et al. (US Pub. Patent Application 2004/0145850) discloses a layer of NixFe1-x where x is controlled below 0.82 to adjust magnetostriction. Stress is also adjusted.
Slaughter et al. (U.S. Pat. No. 7,067,311) discloses choosing the Co to Fe ratio in CoFeB to control magnetostriction.
Saito et al. (U.S. Pat. No. 6,868,002) teaches that magnetostriction can be set to zero by adjusting the ratio of elements in the magnetic film.
Albert et al. (U.S. Pat. No. 6,847,547) shows that a small concentration of non-magnetic materials can be doped into the free layer to optimize magnetostriction.
Li et al. (U.S. Pat. No. 6,809,516) teaches controlling magnetostriction of a FeCo alloy can be achieved by controlling its composition and annealing processes.
Kamiguchi et al. (U.S. Pat. No. 6,052,262) discloses controlling magnetostriction via the composition of a NiFe alloy.
Zhao et al. (US Pub. Patent Application 2007/0047159) teaches controlling the magnetostriction of the free layer by varying the composition or thickness of layers.
Sun (U.S. Pat. No. 6,130,814) describes magnetic moment switching using spin transfer.
Gill (U.S. Pat. No. 6,665,155) describes a combination of a layer with positive magnetostriction and a layer with negative magnetostriction as a means of adjusting the total magnetostriction.
What is needed is a method of applying shape anisotropy, combined with the effects of stress and magnetostriction to spin transfer type MRAM cells to control their overall magnetic anisotropy.